> [Original Message]
> From: Peter Kirk <peterkirk@qaya.org>
>
> On 29/03/2004 06:56, John Cowan wrote:
>
> >Peter Kirk scripsit:
> >
> >
> >
> >>Using NBSP rather than SPACE has several advantages, and has long
> >>been specified in Unicode, although not widely implemented. It is less
> >>likely to occur accidentally. But it has disadvantages, especially that
> >>it will always be a spacing character, whereas for display of isolated
> >>Indic vowels no extra spacing is required.
> >
> >You don't actually say so, but you give me the impression that you think
> >NBSP is a fixed-width space. It isn't; it can assume any width greater
> >than zero, just as SPACE can; in particular, when used before a NSM, I
> >would expect it to have the same width as the NSM.
>
> Well, as I understand it NBSP is often expected to be a fixed-width
> space, and it is in many implementations. In fact I think it ought to
> be, whether or not this is actually specified. But there ought to be a
> character which is explicitly NOT fixed width to carry NSMs. Also
> you do say that NBSP must have a width greater than zero, but for
> some combining marks (those which are not non-spacing, and
> arguably even some which are) this base character should have
> zero width.

UAX #14 makes a rather definitive statement on this issue, albeit
in an obscure place, in Section 3: Introduction.

"When expanding or compressing inter-word space, only the space
marked by U+0020 SPACE and U+3000 IDEOGRAPHIC SPACE
are normally subject to compression, and only spaces marked by
U+0020 SPACE, and occasionally spaces marked by U+2009
THIN SPACE are subject to expansion. All other space characters
have fixed width."

While one can argue as to whether this has anything to do with the
effect on the width of NBSP with a combining character following
it or not, it is clear that clear that one should not assume that NBSP
is treated exactly the same as SPACE except for not breaking a line.
Indeed, I would prefer to see NBSP treated as a fixed-width character
that would only be affected by letter spacing in all contexts, including
when it has an attached combining character.

The idea of an explicit character to be used as a combining
character base has merit in my opinion, but only if an acceptable
standardization of the behavior of combining characters with some
other character such as SPACE cannot be achieved so that it would
always be expected to produce an isolated combining character.
(except when in an intentional show the codes mode)